Functions that map a value from one form to another are useful since you can separate the mapping logic from things like services and components which are best kept as simple as possible. This allows for easy tests where you can check a range of conditions which might take a lot more boilerplate code if the logic was embedded in a component or service. It also allows you to re-use the mapping.
This does lead to simpler test cases, but because they are so simple the test boilerplate can become repetitive and overwhelm what is actually being tested. In practice, these types of tests are often copied and pasted which can lead to more maintenance overhead or outdated commentary around tests. To illustrate, consider the following mapping function.
Transformating Function
The following function takes in a number and returns numbers based on a series of conditional checks. transform.ts
// transform.ts
export function transform(value: number): number {
if (value === undefined) {
return 0;
}
if (value === null) {
return 0;
}
if (value < 0) {
return -1;
}
if (value === 0) {
return 0;
}
return 1;
}
Traditional Tests
To test the function you might come up with cases where the input is undefined, null, -10, -1, 0, 1 and 10. Those seem reasonable given the checks and boundaries in the tests. This would result in something like the following test file. transform.spec.ts
// transform.spec.ts
import { transform } from './transform';
describe('transform', () => {
describe('input value is undefined', () => {
it('should return 0', () => {
expect(transform(undefined)).toEqual(0);
});
});
describe('input value is null', () => {
it('should return 0', () => {
expect(transform(null)).toEqual(0);
});
});
describe('input value is -10', () => {
it('should return -1', () => {
expect(transform(-10)).toEqual(-1);
});
});
describe('input value is -1', () => {
it('should return -1', () => {
expect(transform(-1)).toEqual(-1);
});
});
describe('input value is 0', () => {
it('should return 0', () => {
expect(transform(0)).toEqual(0);
});
});
describe('input value is 10', () => {
it('should return 1', () => {
expect(transform(10)).toEqual(1);
});
});
describe('input value is 1', () => {
it('should return 1', () => {
expect(transform(1)).toEqual(1);
});
});
});
The key pieces of information in the tests is that undefined and null map to 0, negative numbers map to -1, 0 maps to 0 and positive numbers map to 1. Compare that simple sentence to 43 lines of code required to implement the tests.
Lines 7 to 13 clearly demonstrate the expected input to output mapping. Lines 15-19 demonstrate how the mapping is achieved. The forEach loop takes advantage of object destructing to assign an object from the array to parameters of the testing function on line 14. The describe and it functions are passed arguments that include the input and expected value, which makes reading and understanding the output of failed tests easier, on lines 15 and 16, respectively.
There are drawbacks to parametrizing tests and, in a lot of cases, can make tests confusing. In this case, the parameters and code to execute the test fit on the same page in a code editor, which makes it easy to understand. There are also few input parameters and the test logic is simple.
When the number of input parameters gets large, it makes the test harder to read because you have to jump back and forth between the parameter definition and the test code. If the test code is longer and complex, introducing parameters increases the complexity of the test even further. If parametrizing the test means you have to significantly increase the logic of the test, it means the test is hard to understand when you come back to it a few months down the track.
However, with few input parameters and a function for which little test logic is required, using parametrised tests reduces boilerplate code and makes the tests easier to understand. TypeScript also helps as it will infer the types of the input and expected parameters for you.
The service with a subject pattern is a good entry point into the RxJS world of observables in Angular. When you are using RxJS, writing traditional tests can be cumbersome. There are great tools, such as jasmine-marbles, that help make these kinds of tests a lot easier. A great way to get started with jasmine-marbles is testing the service with a subject when the service only performs HTTP calls. However, when the service also accepts updates from components, writing these tests can be more difficult as you have to bridge the gap between the timing of function calls and observables.
The Service under Test
To illustrate a starting point for how to write these kinds of tests, consider a service that supports a component that allows the user to pick the sauce they would like to have on their burger 🍔. You might come up with a sauce service to keep track of which sauce was picked, such as the one shown below. sauce.service.ts
There are two things you may be interested in testing. The first is that the initial sauce is set correctly and the second is that, on calling setSauce, that the new sauce is emitted by sauce$. Both of these can be tested using jasmine-marbles. Without jasmine-marbles, you would have to subscribe to sauce$, check that the correct sauces are emitted and make use of the done function. With jasmine-marbles this can be simplified.
To see how, you need to understand that the jasmine-marbles cold function simply creates an observable. Just like any other observable, you can subscribe to it. So if you define an observable that emits the sauces that you want to call setSauce with using cold, subscribe to that observable and call setSauce with the sauces that you receive, you have bridged the gap between function calls an observables. The marble syntax allows you to specify the timing of the setSauce calls. The tests could like like what is shown below. sauce.service.spec.ts
The test on lines 15-22 checks that the initial sauce is emitted after construction. The interesting test is on lines 26-39. On line 28 the timing of the setSauce calls is defined. In this case, it is setup so that the first setSauce call occurs one frame after the construction is complete. On lines 32-33 the input to the setSauce call is defined and the plumbing to perform the setSauce calls is specified. On lines 36-37 the standard work is being done to check the values emitted by sauce$.
This technique is more concise and easy to understand than messing around with observables, subscriptions and the done function. There are still things that could be better. For example, it would be great if jasmine-marbles could come up with a standardised way of performing this technique (maybe they already have and I just don’t know about it). You may not feel like using this technique all the time, depending on the complexity of the logic and its dependence on timing. However, knowing about it and how it helps you can be valuable when you encounter the need for finer control over timing of function calls in tests.
Previously, you had an idea to move a spreadsheet from an email trail onto a website. To achieve that, you used an Angular Material Basic Spreadsheet. You showed it to your manager and he thought it had potential if you add a few more features. Before going ahead with that, you realise that, to achieve complexity scale, you will have to start writing some tests.
As a reminder, the key elements of the Angular Material Basic Spreadsheet are the component that displays individual orders. This component is used by an Angular Material table based table component that maps a number of orders onto the screen similar to a spreadsheet based on the date and product of the order, as shown below.
// order.component.ts
import { Component, OnInit, Input } from '@angular/core';
import { Order } from './order.model';
@Component({
selector: 'app-order',
templateUrl: './order.component.html',
styleUrls: ['./order.component.css']
})
export class OrderComponent implements OnInit {
@Input() order: Order;
constructor() { }
ngOnInit() {
}
}
<!-- order.component.html -->
<ng-container *ngIf="order">{{ order.value | number }}</ng-container>
To figure out the tests that need to be written, let’s think about what could go wrong. Firstly, we could have forgotten to import something or setup something incorrectly. Next, we could have assumed that anyone that uses the component will always pass in an order. Thirdly, when an order is passed in, we could have forgotten to use the number pipe to display the value. The following specification tests for those three cases. order.component.spec.ts
// order.component.spec.ts
import { TestBed, ComponentFixture } from '@angular/core/testing';
import { OrderComponent } from './order.component';
import { DebugElement } from '@angular/core';
describe('OrderComponent', () => {
let fixture: ComponentFixture<OrderComponent>;
let component: OrderComponent;
let debugElement: DebugElement;
beforeEach(() => {
TestBed.configureTestingModule({
declarations: [OrderComponent]
});
fixture = TestBed.createComponent(OrderComponent);
component = fixture.componentInstance;
debugElement = fixture.debugElement;
});
it('should create', () => {
fixture.detectChanges();
expect(component).toBeTruthy();
});
describe('order null', () => {
it('should display empty template.', () => {
// Setting order to null
component.order = null;
// Triggering change detection
fixture.detectChanges();
// Checking template
expect(debugElement.nativeElement.textContent).toEqual('');
});
});
describe('order not null', () => {
it('should display order value formatted as number.', () => {
// Setting order to null
component.order = {date: '2000-01-01', product: 'product 1', value: 1000};
// Triggering change detection
fixture.detectChanges();
// Checking template
expect(debugElement.nativeElement.textContent).toEqual('1,000');
});
});
});
On lines 8-20 the standard setup for component tests is defined. The test on lines 22-25 checks that the component is created. The test on lines 27-38 checks what happens if the input Order is null. Finally, the test on lines 40-51 checks that the Order value is displayed using the number pipe.
Here the DebugElement is used to inspect the template. Since the template is relatively simple, we only ever check the text content of the whole template under the different test cases.
The table component is more complex and will require more tests. Broadly, there are tests for the component class and for the template. The main test for the component class is that the getOrderTable function is called on initialisation. Ideally, component code is tested without the use of the TestBed as it tends to slow down tests. However, since the component code to be tested is linked to an Angular event (OnInit), in this case it is better to use the TestBed and, therefore, the standard Angular lifecycle functions. The template tests are complex due to the number of ngIf statements and other logic in the template. Additionally, there are some complexities testing Angular Material tables. The following is the test code that checks common cases. order-table.component.spec.ts
The test setup is done on lines 23-41. The tests on lines 43-76 check the code of the component. The tests on lines 78-286 check the template under a range of scenarios.
Test Setup for Angular Material Table
There are a few interesting pieces to highlight for the tests. As the OrderTemplateComponent has a dependency on the OrderComponent, it needs to be given to the TestBed along with the OrderTableComponent. For testing, it is better to define a mock implementation of the component rather than use the real component, which is done on lines 10-20.
At the time of writing, the DebugElement does not work with the Angular Material table. This means that, to figure out exactly which OrderComponent is displayed in a cell, we need to make use of the template. Each OrderComponent displays an Order. Therefore, it should be enough to display all the member variables of the Order in the template of the mock OrderComponent to know that the correct Order was passed.
Since the template of the OrderTableComponent makes use of the Angular Material table, we need to pass in the MatTableModule to the TestBed.
Component Code Tests
The tests on lines 43-76 check that the component can be constructed and that the getOrderTable function is called on the OrderService during component initialisation. In this case, the component create test is a little more complicated because the OrderService needs to be called during initialisation which means that the getOrderTable function must at least exist. Also, during the initialisation, the headers member variable is used on the returned value of getOrderTable, so we may as well return an empty OrderTable.
Component Template Tests
The main reason for the length of the tests are the definition of the OrderTable to display in each test. There is a trade off between pre-defining or repeatedly defining them in each test. In production code brevity is preferred, so it is often better to define a range of OrderTables and importing them. However, for tests, it is better to have a single test be as much of a complete story as is needed to understand the test. As what is displayed in the template is tightly linked to the OrderTable, there are significant advantages to explicitly stating the OrderTable in each test.
The test scenarios are an empty table, a single item table and tests where either multiple columns or rows are displayed. In each case, the headers and rows are checked.
With these tests you have made an important step towards being able to scale up the complexity of the website. Now it is time to work on additional features! The most important feature request your manager has made is to make the spreadsheet editable so that people can revise the orders that were placed over time.
Spreadsheets are the lifeblood of many organisations. For example, a fruit company might be keeping track of the number of orders for Apples and Oranges over time using a spreadsheet. To scale, the organisation might have chosen to create 2 teams, one that looks after orders for Apples and the other for Oranges. Management wants a wholistic view of orders, so it instructs you to gather sales of Apples and Oranges from the two teams and aggregate the sales in a single spreadsheet. A trail of emails starts with many attached spreadsheets to keep track of the sales. Arguments ensue on whether the spreadsheet was updated correctly, who updated it and when.
What if there is a simpler solution? You decide to try building an internal website that keeps track of the sales. As a starting point, you only want the website to display orders broken down by product (Apples and Oranges) and date. You decide to try to use work that other people have already done for you. In particular, you decide to use @angular/material. To focus your mind on what you want to achieve, you draw up a draft of what you would like the website to look like:
Draft of what the website should look like.
Order Model
The first step is to define a model for an order. A simple model would keep track of the product, the date of the order and the amount that was ordered. order.model.ts
// order.model.ts
export class Order {
date: string;
product: string;
value: number;
}
Displaying the Order
To get a feeling for the model, you decide to write a component that can display a single Order. For now, it will be fairly basic and only display the value of the order, formatted as a number. order.component.ts order.component.html
// order.component.ts
import { Component, OnInit, Input } from '@angular/core';
import { Order } from './order.model';
@Component({
selector: 'app-order',
templateUrl: './order.component.html',
styleUrls: ['./order.component.css']
})
export class OrderComponent implements OnInit {
@Input() order: Order;
constructor() { }
ngOnInit() {
}
}
The component code is simple, it just accepts an Order as input.
In the template, you decide to check whether the order is truthy (ie. it has actually been passed in) using the *ngIf statement on line 2 and only display the value in that case.
Order Table Model
You anticipate the need to display multiple orders in a table. This table needs to be displayed by the Angular Material table component. To keep the template as simple as possible, you decide to use the following table model. order-table.model.ts
// order-table.model.ts
import { Order } from './order.model';
export class OrderTable {
headers: string[];
rowLabels: string[];
orders: Order[];
orderMap: {[header: string]: {[rowLabel: string]: Order}};
}
It is designed to keep track of the orders in the table using the orders member variable. The ordering of the headers and rows is defined by headers and rowLabels member variables, respectively. To link a header and row label to and Order, you use the orderMap member variable.
Order Service
As you still need to convince your management and the teams you work with that the website is a good idea, for demonstration purposes, you decide to create a service that can easily be extended in the future, but that returns a hardcoded table for now. order.service.ts
For the moment it returns 3 static orders, 2 for Apples and 1 for Oranges.
Displaying the Order Table
To be able to use @angular/material, you need to install some packages as shown in the getting started guide. To be able to use the Angular Material table, you need to import the MatTableModule and put it into the imports array of the module which will display the table.
The component code needs to make use of the OrderService to get the order table. It also needs to define the heading for the product name. The component template needs to dynamically generate headings and rows, depending on the number of Orders that need to be displayed. table.component.ts table.component.html
Line 14 defines the header under which the product will be displayed. In the component initialiser on line 20, the OrderTable is loaded. On line 21 the headers of the table are concentrated with the header for the product.
The table template is the most complicated piece of the puzzle. Line 2 defines that the data source for the table are the row labels of the table. This allows you to dynamically generate a row for each row label in the table. Line 14 defines how the header columns are defined and line 15 defines how each row puts data into the columns. Line 3 is the generator expression for each column of the table.
Lines 4-7 define how the header row is generated. Line 5 checks whether the current header is the product header, and if so displays the product header. Line 6 checks the opposite and displays the header after passing it through the date pipe.
Lines 8-11 define how the data rows are generated. Similar to the header row, line 9 checks for the product header and then displays the product. Line 10 checks for the opposite, picks out the Order based on the header and row label using the orderMap and passes on the Order to be displayed by the OrderComponent.
Now you have a working website that you can take to your management and the teams you work with as a showcase. They will probably ask you for additional features, such as being able to edit orders and add new orders to the table. To be able to deliver those more complex features, you also start thinking about how you might write tests for what you have created so far.
The Service with a Subject pattern in Angular is a good way to get started with reactive programming (ie. making use of RxJS) in Angular. The following article describes how to get started with the pattern: Getting Started with Service with a Subject in Angular.
That pattern leads to writing reactive components for which standard Angular component testing would lead to more complicated tests. With some changes to the standard Angular component testing methodologies, the tests can be simplified significantly.
Reactive Component under Test
For reference, the following is the typescript and HTML template for the component. name.component.ts
// name.service.ts
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { BehaviorSubject, Observable, of } from 'rxjs';
@Injectable({
providedIn: 'root'
})
export class NameService {
private mName$ = new BehaviorSubject<string>(null);
name$(): Observable<string> {
return this.mName$;
}
constructor(private httpClient: HttpClient) { }
loadName() {
this.httpClient.get<string>('name URL')
.subscribe(
name => this.mName$.next(name)
);
}
}
To figure out what needs to be tested, let’s think about what could go wrong. There are 3 main things that are likely to go wrong. The first is that the component doesn’t even get created due to some syntactical or import error. The second is that the loadName function on the NameService doesn’t get called during initialisation. The third is that, once the NameServicename$ function emits a name, it doesn’t get displayed in the component.
Reactive Component Tests
The following is the test code that checks the 3 things that are likely to go wrong described earlier. name.component.spec.ts
import { ComponentFixture, TestBed } from '@angular/core/testing';
import { of } from 'rxjs';
import { NameService } from './name.service';
import { NameComponent } from './name.component';
describe('NameComponent', () => {
let nameServiceSpy: jasmine.SpyObj<NameService>;
let component: NameComponent;
let fixture: ComponentFixture<NameComponent>;
beforeEach(() => {
nameServiceSpy = jasmine.createSpyObj('NameService', ['loadName', 'name$']);
TestBed.configureTestingModule({
declarations: [ NameComponent ],
providers: [ { provide: NameService, useValue: nameServiceSpy } ]
});
fixture = TestBed.createComponent(NameComponent);
component = fixture.componentInstance;
});
it('should create', () => {
// Checking component creation
expect(component).toBeTruthy();
});
it('should call loadName on NameService', () => {
fixture.detectChanges();
// Checking loadName call
expect(nameServiceSpy.loadName).toHaveBeenCalledWith();
});
it('should display NameService name', () => {
// Setting name in service
nameServiceSpy.name$.and.returnValue(of('the name'));
fixture.detectChanges();
// Checking displayed name
expect(fixture.debugElement.nativeElement.innerText).toEqual('the name');
});
});
Test Setup
The setup common to all tests is on lines 8-21 which define the component under test and other supporting variables needed for he tests. Lines 13-21 define the standard TestBed initialisation and injection logic. The three test scenarios are defined on lines 24-27 (checking the component can be created), 29-34 (checking that loadName is called on NameService) and 36-44 (checking that the name emitted on name$ is displayed).
Component Setup
The test on lines 24-27 checks that the component can be setup. This is a simple test but is a very useful step for verifying that all the component plumbing is correct and that the test setup has actually worked. If this test fails it is likely because an injection is missing or because the TestBed was not setup correctly.
loadName Call
The test on lines 29-34 checks that the loadName function is called on the NameService. For the ngOnInit function to be triggered, change detection is run on line 30. Line 33 checks that loadName has been called on the NameService.
Name Display
The test on lines 36-44 checks that, when name$ on NameService emits a new name, that name is displayed in the component. Line 38 triggers emitting a new name on name$. Line 40 triggers change detection so that the name is displayed. Line 43 checks that the name is displayed in the component.
These tests are the base on which tests can be built if the logic in the component becomes more complicated. Examples of additional complications are having to pass arguments to the service loadName call, displaying data from multiple services or subjects and a more complicated template that includes conditional displaying of data.
The Service with a Subject pattern in Angular is a good way to get started with reactive programming (ie. making use of RxJS) in Angular. The following article describes how to get started with the pattern: Getting Started with Service with a Subject in Angular.
The next step after implementing the Service with a Subject pattern is to write tests for the service. These tests will involve working with observables. Testing observables with plain Jasmine leads to tests with logic not related to verifying that the service works. The jasmine-marbles library includes several methods that help test observables.
The Service with a Subject Under Test
For reference, the following is the service that needs to be tested. name.service.ts
// name.service.ts
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { BehaviorSubject, Observable, of } from 'rxjs';
@Injectable({
providedIn: 'root'
})
export class NameService {
private mName$ = new BehaviorSubject<string>(null);
name$(): Observable<string> {
return this.mName$;
}
constructor(private httpClient: HttpClient) { }
loadName() {
this.httpClient.get<string>('name URL')
.subscribe(
name => this.mName$.next(name),
() => null
);
}
}
To figure out what needs to be tested, let’s think about what could go wrong. There are 3 main things that are likely to go wrong. The first is that the incorrect URL is passed to the get function of the HttpClient. The second is that the return value of the HTTP GET request is not passed to the subject correctly. The third is that, if the HTTP GET request returns an error, an exception is raised.
Service with a Subject Tests
The following is the test code that checks the 3 things that are likely to go wrong described earlier. name.service.spec.ts
// name.service.spec.ts
import { HttpClient } from '@angular/common/http';
import { cold } from 'jasmine-marbles';
import { NameService } from './name.service';
describe('NameService', () => {
let service: NameService;
let httpClientSpy: jasmine.SpyObj<HttpClient>;
beforeEach(() => {
httpClientSpy = jasmine.createSpyObj('HttpClient', ['get']);
service = new NameService(httpClientSpy);
});
describe('loadName', () => {
it('should call get on HttpClient with correct URL', () => {
// Setting up get spy
httpClientSpy.get.and.returnValue(cold('a|', {a: null}));
// Calling loadName
service.loadName();
// Checking get call
expect(httpClientSpy.get).toHaveBeenCalledWith('name URL');
});
it('should emit new name on name$', () => {
// Defining marbles
const getSpyMarbles = '-a|';
const expectedMarbles = 'bc';
// Setting up get spy
httpClientSpy.get.and.returnValue(cold(getSpyMarbles, {a: 'name'}));
// Calling loadName
service.loadName();
// Checking get call
const expected = cold(expectedMarbles, {b: null, c: 'name'});
expect(service.name$()).toBeObservable(expected);
});
it('should emit nothing on name$ if an error occurs', () => {
// Defining marbles
const getSpyMarbles = '-#|';
const expectedMarbles = 'a';
// Setting up get spy
httpClientSpy.get.and.returnValue(cold(getSpyMarbles));
// Calling loadName
service.loadName();
// Checking get call
const expected = cold(expectedMarbles, {a: null});
expect(service.name$()).toBeObservable(expected);
});
});
});
Test Setup
The setup common to all tests is on lines 8-14. Lines 8 and 9 define the service under test and the HttpClient spy used for the tests that are initialised on lines 11-14. The three test scenarios are defined on lines 17-26 (checking the URL passed to the HttpClientget function), 28-41 (checking that the return value of the HTTP GET request is passed on) and 43-56 (checking the behaviour of the service when the HTTP GET returns an error).
GET Request URL
The test on lines 17-26 checks that the correct URL is passed to the get function of the HttpClient. The first step on line 19 is to give the get call a return value. In this case we only care about what is passed to the get function, so what is set as the return value is irrelevant as long as it is an observable. Line 22 triggers the function that triggers the GET request. Line 25 checks that the get function was called with the correct URL.
Return Value is Passed On
The test on line 28-41 checks that the HTTP GET request return value is passed on by the name$ function as an observable. Lines 30 and 31 define marbles that control when the HTTP GET request emits a value and when the name$ function should emit a value, respectively. Line 33 defines what the HTTP GET request returns.
Line 36 triggers the function that triggers the HTTP GET request. Line 39 defines that value that is expected to be emitted by the name$ function (being the null with which the BehaviourSubject is initialised and the GET request return value) and line 40 checks for that expectation.
HTTP GET Request Error
The test on lines 43-56 checks the behaviour of the code if the HTTP GET request returns an error. Lines 45 and 46 define marbles that control when the HTTP GET request returns an error (indicated by #) and when the name$ function should emit a value, respectively. Line 48 defines what the HTTP GET request should return.
Line 51 triggers the function that triggers the HTTP GET request. Line 54 defines that value that is expected to be emitted by the name$ function (being only the null with which the BehaviourSubject is initialised) and line 55 checks for that expectation.
These tests are the base on which tests can be built if the logic in the service becomes more complicated. Examples of additional complications are if the HTTP GET return value needs to be transformed or if multiple HTTP GET calls need to be made that are potentially merged together.
A powerful technique in Angular is the built in integration with RxJS. Making use of reactive programming (ie. RxJS) can remove a lot of code from your Angular application. This is an indictor that reactive programming takes care of a lot of the logic you would otherwise have to come up with and test yourself.
There are simple steps you can take to get started with writing reactive Angular applications. You can then use the understanding and confidence you get from the simple steps to start using reactive programming in other, more complicated cases.
Simple Service with a Subject
The following code is a simple Angular service with a subject. name.service.ts
There are 3 pieces to the subject that come together to trigger getting the name from the API and passing on the retrieved name.
API Call
The entry point for a user of the service is the loadName function on line 18. It triggers the HTTP call to the API and passes on the retrieved name to mName$ in the subscribe call on line 20.
Storing the Retrieved Name
The mName$ member variable on line 10 of the service is used as private intermediate storage of the name retrieved from the API in the service. mName$ is tightly linked to the name$ function which is used to control access to the intermediate storage.
Passing on the Retrieved Name
The name$ function of the service is used to pass on the name retrieved from the API on line 12. Hiding the private intermediate storage mName$ behind the name$ function allows control over how the name is distributed throughout the Angular application.
The next step is to show the name in a component.
Reactive Component
Once the name has been retrieved from the API, the next step is to show it in a component.
Component Typescript Code
The following is the typescript code for the component. name.component.ts
There is hardly any code in the component that is not part of the required Angular component code. The only line that has been added is line 15 which instructs the service to load the name along with importing the service on line 4 and injecting it into the component on line 12.
Component Template
The following is the HTML template associated with the component. name.component.html
<!-- name.component.html -->
<ng-container *ngIf="nameService.name$() | async as name">
{{ name }}
</ng-container>
In the template the name$ function of the service is passed to the async pipe on line 2. The values emitted by the name$ observable are renamed to the name variable and then displayed on line 3.
That concludes the simple example for making use of a subject with a service. There are some additions to consider depending on the use case. For example, you may want to introduce error handling in the subject. You may also want to write tests for the service and the component.
David Andersson 